The invention is directed to an apparatus for storing self heating radioactive materials, especially for the storage of irradiated fuel elements from nuclear reactors, which in a given case are enclosed in containers. This apparatus consists essentially of a reinforced concrete shell having inlet and outlet air shafts for natural convection of the cooling medium and one or more storage racks for receiving the radioactive materials.
Self-heating radioactive waste, as e.g. burned up fuel elements from pressure and boiling water reactors, at present are preferably stored under water in order to decrease the activity of the fission and activation products to decay. The water hereby fulfills simultaneously the functions of the shielding of the radioactive irradiation and the cooling of the hot fuel elements or the waste containers.
The cooling of the fuel elements is necessary in order to lead off the liberated decay heat the amount of which is dependent upon the size of the burn-up in the reactor and the cooling time already elapsed.
The heat is carried off from the cooling water to the environment for the most part in external coolers through a secondary cooling water circuit and a wet cooling tower. Due to the small heating up stretches available there result relatively large cooling water throughputs and large cooling surfaces. The storage of burned-up fuel elements from nuclear power plants in water tanks therefore has the disadvantage that there occurs a high consumption of cooling water, the environment is molested through the cooling towers and the purification of the water as well as the intermediate and final treatment of the separated radioactive waste is expensive.
Besides there is necessary a high expense for sealing and therewith expense for safety for the water tanks since the water can be radioactively contaminated through leaks which cannot be entirely avoided in the fuel element jackets and additionally the unavoidable radiolysis of the water basis must be controlled.
Furthermore with accidents which could lead to a lost of the cooling medium there is no longer guaranteed sufficient removal of heat so that in addition extensive reserve cooling systems are necessary.
Therefore there have also been known proposals to insert radioactive waste in so-called dry storage in which a gas preferably air, is used as cooling medium that carries off the heat from the stationary stored material in the storage racks by forced cooling with, e.g. fans over heat exchangers or directly to the surroundings. However, there is the disadvantage thereby that in accidents, i.e. breakdown of the cooling system or the cooling aggregate there is no longer guaranteed sufficient transportation of heat which can lead to inadmissible increases in temperature and to the setting free of toxic radioactive materials.
For these reasons dry storage was developed in which the heat is led off through natural convection. These systems are inherently safe since, because of the natural convention, they do not require any active components or operating aggregate to maintain the cooling operation. There is known an apparatus (German OS No. 2730729 and related Klein U.S. application Ser. No. 922,352 filed July 6, 1978) in which the heat of the active waste is led off through natural convection in a closed cell on heat exchanger walls and likewise in an outer cooling system gives up heat to the surroundings through natural convection (indirect cooling).
From German OS No. 2711405 (and related Pirk U.S. application Ser. No. 884,818 filed Mar. 9, 1978) there is known an apparatus in which the heat of the radioactive waste is led off through natural convection directly to the air of the surrounding (direct cooling). In both apparatus the encased stored material is inserted into concrete chambers in vertical shafts through which the cooling medium air flows upward through the stored material because of the heating and thus leads off the heat.
The vertical storage of self heating radioactive wastes which above all is dependent on the available transportation and storage devices has the disadvantage that substantially higher temperatures occur in the upper end of the shafts than in the lower. Besides with multiple charging of the individual shafts the freshly stored containers giving off, especially large amounts of heat normally are stored uppermost in the shafts where the poorer cooling conditions prevail. With indirect cooling there is customarily a disparity between primary heat exchanger surfaces (sum of the surface areas of the stored materials in the shafts) and secondary heat exchange surfaces (effective surfaces of the walls). There is obtained thereby a relatively high temperature level at the stored material. Furthermore, with the vertical storing relatively large incoming air and outgoing air shafts and openings are necessary which for reasons of industrial safety must be equipped safe from sabotage, airplane collision and earthquake.
Therefore it was the problem of the present invention to develop an apparatus for storing self heating radioactive materials, especially irradiated fuel elements, which in a given case are enclosed in containers, in which there is attainable a safe cooling even in the case of accident, a most uniform temperature distribution along the container containing the radioactive materials and above all a safe arrangement of the incoming and outgoing air openings with the smallest possible cross section. Thereby the apparatus should consist essentially of a concrete chamber with incoming and outgoing air shafts for natural convection of the cooling medium and one or more storage supports for receiving radioactive materials.